1. Field of the Invention
The present invention relates to a bandgap circuit. More particularly, the present invention relates to a bandgap circuit of a current mode and a voltage mode.
2. Description of Related Art
A bandgap circuit is used for generating a stable reference voltage that is not influenced by temperature variation. FIG. 1 is a circuit diagram illustrating a conventional current mode bandgap circuit. In which, metal oxide semiconductor field effect transistors (MOS transistors) M1, M2 and M3 form a current mirror to equalize currents I1, I2 and I3. Two input terminals of an operation amplifier OPA respectively receive input voltages VIN and VIP, and the input voltages VIN and VIP can be equalized by a virtual short circuit effect of the operation amplifier OPA. Resistors R1 and R3 have a same resistance, and the input voltages VIN and VIP are equal, so that currents flowing through the resistors R1 and R3 are the same, and accordingly currents flowing through bipolar junction transistors (BJTs) Q1 and Q2 are the same. As shown in FIG. 1, a size of the BJT Q2 is x times greater than that of the BJT Q1, in this case, a voltage difference between emitters of the BJTs Q1 and Q2 is VTLnX. Wherein, VT presents a thermal voltage, and VT=kT/q, wherein k is a Boltzmann's constant, T represents a current absolute temperature, and q represents a quantity of electrical charge 1.6×10−19 coulombs, and Ln represents a natural logarithm. Namely, a voltage formed between two ends of the resistor R2 is VTLnX.
According to the above conditions, an amount of the current I2 is (VTLnX)/R2+VEB1/R3, wherein VEB1 represents a voltage between the emitter and a base of the BJT Q1. Since the currents I2 and I3 are the same, a bandgap reference voltage VBG provided by the circuit of FIG. 1 is [(VTLnX)/R2+VEB1/R3]*R4. The thermal voltage VT has a positive temperature coefficient, and the voltage VEB1 has a negative temperature coefficient. As long as values of X, R2 and R3 are suitably designed, the positive temperature coefficient and the negative temperature coefficient can be counteracted, so that the currents I1, I2 and I3 are not influenced by the temperature variation, and accordingly the bandgap reference voltage VBG is not influenced by the temperature variation.
The operation amplifier OPA can apply an NMOS transistor input structure as that shown in FIG. 2, and can also apply a PMOS transistor input structure as that shown in FIG. 3. Regarding the NMOS transistor input structure of FIG. 2, the input voltages VIN and VIP has to be great enough to normally operate the operation amplifier OPA. Namely, a following condition has to be satisfied:VEB1>VTHN+VDS15 
Wherein, VTHN is a threshold voltage of an NMOS transistor M11, and VDS15 is a voltage between a drain and a source of an NMOS transistor M15 when the NMOS transistor M15 is operated in a saturation region. A problem is that if the threshold voltage VTHN is too high, within a system temperature range, the threshold voltage VTHN is probably greater than the input voltage VEB1 throughout, so that the operation amplifier OPA is unable to work.
On the other hand, regarding the PMOS transistor input structure of FIG. 3, a supply power PCC has to be great enough to normally operate the operation amplifier OPA. Namely, a following condition has to be satisfied:VCC>=VEB1+|VTHP|+VDS15 
Wherein, VTHP is a threshold voltage of a PMOS transistor M11. As a fabrication process of a present semiconductor circuit becomes finer, the supply power VCC is accordingly decreased. If the threshold voltage |VTHP| is too high, within the system temperature range, VEB1+|VTHP| is probably greater than the supply voltage VCC throughout, so that the operation amplifier OPA is unable to work.
FIG. 4 is a circuit diagram illustrating another conventional current mode bandgap circuit. To resolve the working problem of the aforementioned operation amplifier OPA, resistors R5 and R6 are further applied to the bandgap circuit of FIG. 4 to promote the input voltages VIN and VIP of the operation amplifier OPA. Resistances of the resistors R5 and R6 are the same, and by using the operation amplifier OPA of the NMOS transistor input structure, as long as the input voltages VIN and VIP are promoted to be greater than VTHN+VDS15, the operation amplifier OPA can normally work. However, since variation of the fabrication process cannot be totally controlled, the PMOS transistors M1 and M2 of the current mirror are probably not totally matched, so that the current I1 is slightly different to the current I2, and the resistors R5 and R6 are probably not totally matched. The above unmatched problem can result in a difference between the two input voltages VIN and VIP of the operation amplifier OPA, which may bring an adverse effect to the bandgap reference voltage VBG.